Electronic transport at the junction between different materials is governed by the band structure line-up between the respective materials and their relative chemical potentials. Misalignment between bands leads to electrical potential barriers that must be overcome for carrier transport to occur. Moreover, equilibration of the chemical potentials occurs when the materials are brought into contact, and results in the formation of a space charge layer that is depleted of free carriers that cause additional barriers to transport and generally degraded device performance.
Low resistance Ohmic contacts to both n and p-type semiconductors are generally required for proper device operation. High contact resistance results in a substantial voltage drop at the contact which can lead to both ac and dc performance degradation, such as switching speed and voltage swing, respectively. Moreover, high contact resistance can lead to reliability degradation through associated Ohmic heating.
In some applications, the electrical contact must also be optically transparent. For example, low resistance transparent p-Ohmic contacts are generally required for GaN light-emitting diodes and laser diodes. One of the lifetime-limiting factors in GaN laser diodes has been the p-Ohmic contact. Due to the relatively high specific contact resistances (rc) achievable, the metallization heats-up as current flows across the p-n junction, leading to metal migration down threading dislocations in the GaN layers and eventual shorting of the junction. There are a number of contributing factors to the high rc values for contacts to p-GaN, including:                (i) The absence of a metal with a sufficiently high work function. The bandgap of GaN is 3.4 eV, and the electron affinity is 4.1 eV, but metal work functions are typically ≦5 eV.        (ii) The relatively low hole concentrations in p-GaN due to the deep ionization level of the Mg acceptor is about 170 meV.        (iii) The tendency for the preferential loss of nitrogen from the GaN surface during processing, which may produce surface conversion to n-type conductivity.        
An additional desirable property of the contact metallurgy in GaN light-emitting diode structures is a high transmittance for visible light, in order to maximize the light output from the top surface of the LED. A general approach in this case is to use a very thin metal contact on the p-layer.
In the search for improved electrical contacts, a wide variety of metallizations have been disclosed. Again with reference to GaN based LEDs, standard Ni/Au, Ni, Au, Pd, Pd/Au, Pt/Au, Au/Mg/Au, Au/C/Ni, Ni/Cr/Au) and Pd/Pt/Au have been disclosed. Typically, at the contact, Ni, Pd or Pt metal is in direct contact with the P—GaN, and the structure is annealed at 400-750° C. This produces contact resistances in the 10−1 to 10−3 Ω-cm2 range. For higher temperatures, severe degradation in contact morphology is observed, usually resulting from the formation of the metal gallides.
Indium-tin-oxide (ITO) has been used as the transparent bottom electrode in GaN ac thin film electroluminescent devices and has been used an overlayer on transparent Ni/Au contacts on p-GaN to lower contact resistance. In general, ITO contacts alone show rectifying behavior on p-type GaN, even after annealing, and is more typically used as an Ohmic transparent layer on n-GaN.